The invention relates to a method of manufacturing a semiconductor device whereby a photoresist layer is provided on a surface of a slice of semiconductor material, after which photomasks corresponding to mutually connecting portions of a pattern to be formed in the photoresist are projected onto the photoresist by means of a projection lens, with pattern edges superimposed on one another, while strip-shaped transparent end portions of the photomasks situated within this edge and mutually overlapping in projection are provided with strip-shaped connection patterns which overlap one another in projection and which exhibit a complementary transmittance in projection.
In such a method, a pattern which is too large for being projected by means of a single photomask onto a layer of photoresist is subdivided into mutually adjoining partial patterns which are accommodated in different photomasks. These photomasks are projected onto the photoresist with overlapping edges each time, so that the portions of the pattern to be formed merge into one another again in projection. Integrated circuits may thus be manufactured which cover a comparatively large surface area of the semiconductor slice. The total pattern in the photoresist layer is then formed by the transparent end portions which overlap in projection and lie inside the edges and by the transparent end portions of the masks which do not overlap in projection and are not situated within the edges.
EP-A-434142 discloses a method of the kind mentioned in the opening paragraph whereby strip-shaped transparent end portions of the photomasks overlapping one another in projection are provided over their entire surface areas with connection patterns having a transmittance which shows a gradient seen in the longitudinal direction of the strip-shaped transparent end portions. This is achieved in that these strip-shaped transparent end portions are provided with non-transparent regions whose number and/or size changes in the longitudinal direction of the strip-shaped end portions and which are not imaged individually in projection.
Were the photomasks not provided with such connection patterns, then the photoresist layer would receive a total radiation dose at the area of the transparent portions overlapping one another in projection which is twice the radiation dose at the area of transparent portions of the photomasks not overlapping one another in projection. The transparent portions which overlap in projection would then receive a double radiation dose. With photomasks projected on a positive photoresist, a pattern will be formed therein after development which shows a greater width where it was irradiated with the double dose compared with where it was irradiated with the single dose. In the case of projection on a negative photoresist, a pattern with a smaller width is formed by double irradiation. Wherever the masks overlap, a widening or narrowing of lines will occur, so that the photomask patterns do not merge seamlessly into one another. Since the connection patterns of the photomasks show a complementary transmittance in projection, it is achieved that the photoresist layer does receive an equally large total radiation dose at the areas of strip-shaped transparent portions overlapping one another in projection during the projection of the photomasks, compared with areas where transparent portions of the photomasks do not overlap in projection. The photoresist is accordingly irradiated with exactly the same radiation dose over the entire surface area of the slice. The patterns of the photomasks will thus merge into one another seamlessly, i.e. without widening or narrowing of lines.
A photomask suitable for being projected onto a layer of photoresist by means of a projector in practice comprises a glass plate covered with a metal layer, such as chromium, into which a pattern of transparent portions has been etched. An electron beam writes the pattern into a photoresist layer provided on the metal layer. After development, a mask will have been formed which is used for etching the metal layer into the desired pattern. The electron beam is controlled by a computer during writing from a computer data file in which the pattern was laid down. A pattern thus provided on a glass plate is projected onto the photoresist layer in practice by means of a projection lens on a reduced scale, for example, reduced by a factor three or five.
The computer data files in which the photomasks have been laid down and which are used in the known method as described comprise a huge quantity of data necessary for defining the connection patterns. It may be necessary in practice for very many, for example 5000 lines of the masks to merge into one another in projection. This may lead to an undesirably large quantity of computer data when the known connection patterns are used.